[issue16399] argparse: append action with default list adds to list instead of overriding

[paul j3]

paul j3 added the comment:

The test file, test_argparse.py, has a test case for this:
'class TestOptionalsActionAppendWithDefault'

    argument_signatures = [Sig('--baz', action='append', default=['X'])]
    successes = [
        ('--baz a --baz b', NS(baz=['X', 'a', 'b'])),
    ]

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nosy: +paul.j3

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<http://bugs.python.org/issue16399>
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